Conventionally known apparatuses of such a type are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. S62-113829, H4-241730, and 2005-146906. Each of these apparatuses is configured to switch its supercharging mode by changing the states of supply of exhaust gas to a high-pressure turbo charger and a low-pressure turbo charger.
Specifically, in the apparatus disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-146906, there are provided a bypass passage for bypassing the turbine of a high-pressure turbo charger, and an exhaust gas bypass valve for adjusting the flow rate of exhaust gas flowing through the bypass passage.
In such an apparatus, the opening of the above-mentioned exhaust gas bypass valve is adjusted in accordance with the operation state of an engine. Thus, the supercharging mode is switched. That is, within a range in which the engine rotational speed is low, supercharging is performed mainly by a small-sized, high-pressure turbo charger which is suitable for operation within a range of small flow rate (twin supercharging mode). Meanwhile, within a range in which the engine rotational speed is high, supercharging is performed by a large-sized, low-pressure turbo charger which is suitable for operation within a range of large flow rate (single supercharging mode).